The present invention generally relates to a recordable information storage medium (e.g. CD-R medium), such as a compact disc, which cannot be eroded, with high security registration and legally secure.
Typically, a CD-R medium consists of a flat polycarbonate disk, coated on one of its main faces with a light sensitive film. By irradiating the film with a light beam emitted, for example, by a laser diode of 10 mW (called the writing diode), local transformations of said film are produced. These transformations may induce a measurable change in the physical properties of the film, in particular its optical reflectivity. Such latter change is particularly exploited for both, incorporating information on the CD-R and then reading it.
The information itself is inscribed on the film in a digital form as a coded series of circular zones or spots of submicron size, treated and processed by laser irradiation, Usually, these spots are distributed along an elliptical spiral track which is drawn on the disk using a standard scanning system known in the prior art.
In order to be usable, the CD-R should have the two following essential characteristics:                a) sensitivity of the pristine film to the light that is emitted by the writing laser diode, thus promoting an optically detectable change of the film, thereby forming spots produced by the laser treatment. Such a film change may be generated by various processes: melting, bleaching, evaporation, creating roughness, ablation, crystallization of an amorphous film or changing the chemistry of the film, for example upon the synthesis of a compound;        b) morphological stability of the film relative to ambient conditions, such as temperature, visible light, humidity, magnetic fields or mechanical disturbance (with a tool or upon handling), the said stability relating to both the spots and the film surface which is not treated by the laser.        
After inscribing the spots and thereby recording the information, the latter must be read by optical means. To this end, the light emitted by a diode (called the reading diode) or an LED (i.e. a light-emitting device) is scanned along the elliptical track and, simultaneously, the light which is reflected by the film is detected and recorded. The beam emitted by the reading diode is continuous and monochromatic. It operates at a wavelength λR ranging from 500 to 750 nm, i.e. at a photon energy comprised between 2.4 eV and 1.65 eV. The beam intensity of the reading diode is much lower than that of the writing diode, typically of the order of 1 mW. Any local morphological change of the film must produce a local change of the reflectivity of the film. That change is measured and detected with the reading diode. If this change can be evaluated, recording of the light which is reflected by the film allows the reading of the information after decoding the digitized information.
Usually, one defines a reading contrast C between the laser-treated spots and the pristine zones of the film is defined as:C=(Rv−Rs)/(Rv±Rs),
where:                RV denotes the value of the reflectivity (measured at λR) of the pristine film and        RS denotes the reflectivity value (measured at λR) of the spots.        
The magnitude of this contrast C is important for assessing the ability of the CD-R to record the information.
However, the long-term preservation of the recorded information is much more problematic, with heavy consequences, than the amplitude of this contrast alone.
Indeed, any material that would easily undergo a transformation by absorbing a small amount of light energy may as well be sensitive to small changes in environmental conditions (e.g. temperature).
In other words, sensitivity to light and morphological stability act in opposite directions: writing at a low energy does not predispose to long-term stability.
Current CD-R's have a film which is highly sensitive to light. Such a film consists in general of a mixture of organic species (chemicals, polymers). Recording information on such films is easily accomplished by transforming the film locally, for example by changing its color (and its reflectivity) or by ablating the film upon absorption of light and absorbing the resulting heat. The optical contrast C which is thus obtained is typically about 0.03 and contemporary CD-R light engravers (so-called burners) are actually set to read such a contrast.
Although this sensitivity to light has the advantage of allowing an easy information recording, it results in a higher fragility of the current CD-R's, especially against heat, as soon as the media is exposed to ambient light for a while.
Being sensitive to ambient in an unpredictable manner, these media are not considered to be safe in the long term. This is particularly critical for important information which must be stored in conditions of absolute safety, in particular medical, legal or military data.
The main consequence of this situation is that recording on current CD-R's is not entitled to legal legitimacy.
Another consequence is that re-recording at close intervals is required for the same information yet without ensuring its safety.
There is therefore a need for high reliability CD-R's on which the recorded information would remain completely insensitive to ambient conditions for extensive periods of time (e.g. exceeding twenty years), so that such recording could be regarded as permanent.